One of the major problems encountered in the development of prosthetic devices that contact blood is non-compatibility of blood with artificial materials. Blood deposits soluble and cellular components on surfaces upon contact. Such deposition and build up of proteins and cells creates serious problems. In this project, the effect of a particular protein which is present in human blood platelets (platelet factor 3 or PF3) is being examined carefully. In previously published studies, it has been shown that PF3 promotes clotting of blood and because of this property, PF3 may play an important role in blood-material interactions. PF3 has been isolated and purified. Using purified preparations, antibodies were prepared against PF3. Attempts are currently in progress to work out a sensitive assay for PF3 so that presence of PF3 in solution or in adsorbed state on materials can be detected and, if possible, quantitated. Using this assay, the role of PF3 in blood-material interaction will be explored. During the last few months we have observed that antihuman PF3 antibody (raised in rabbits) would neutralize clot promoting activity of PF3. We have also determined that clot promoting activity is associated with a portion of PF3. Currently, attempts are being made to purify that component of PF3 which has procoagulant activity and then examine its effect on blood-material interactions.